Shares of Vivus Inc fell sharply on Thursday afternoon after well-known short-seller Citron Research cast doubts on the drugmaker's ability to protect its weight-loss pill's patents.

Citron in a report on its website pointed out the possibility that Johnson & Johnson could challenge Vivus in court over patents that cover usage of anti-seizure drug topiramate -- one of the key ingredients in Vivus' freshly approved Qsymia.